1. Field of Invention
This invention relates to the conversion of chemical energy to electrical energy. In particular, the present invention relates to a new sandwich cathode design having a first cathode active material of a relatively low energy density but of a relatively high rate capability sandwiched between two current collectors and with a second cathode active material having a relatively high energy density but of a relatively low rate capability in contact with the opposite side of the current collectors. The current collectors are configured so that they only overlay or are vertically aligned one above the other in a middle zone of the electrode assembly. This middle zone extends approximately the same length as a flat winding mandrel that will later be used to wind the sandwich cathode and a lithium anode into a so called flat-folded wound electrode assembly. Such an electrode assembly is then housed in a prismatic shaped casing.
Providing the first and second current collectors only vertically aligned with each other in a middle zone where the winding begins means that as the electrode assembly is wound, the respective cathode active materials do not de-laminate from the current collectors. This is because there is no place in the wind where the two current collectors experience different turn radii. The present sandwich cathode design is useful in applications where a premium is places on increased energy density, such as in power sources associated with implantable medical devices.
2. Prior Art
The use of a sandwich cathode design with dual current collectors is described in U.S. Pat. No. 6,551,747 to Gan. This patent describes a sandwich cathode having a first cathode active material of a relatively high energy density but of a relatively low rate capability, for example CFx, sandwiched between two current collectors with a second cathode active material having a relatively low energy density but of a relatively high rate capability, for example SVO, in contact with the opposite sides of the current collectors. Such an exemplary cathode design has the following configuration: SVO/first current collector/CFx/second current collector/SVO.
Up to now, this cathode configuration has been built in plate-type designs. However, plate-type designs are impractical for a wound cell because the current collectors extend the entire length of the active materials. A cathode comprising two current collectors extending along the entire electrode length cannot be easily wound because the turn radii of two current collectors are not the same. Consequently, the current collectors tend to separate from the cathode materials causing de-lamination. This means that prior to the present invention use of the sandwich cathode designs described in the Gan patent was limited to plate-type configurations. Such so called multi-plate designs are less efficient to manufacture than the wound electrode designs of the current invention.